In recent years, fuel cells wherein hydrogen is supplied to an anode and air which contains oxygen is supplied to a cathode to generate power are being developed energetically. In a system which uses such a fuel cell as just described, a valve device having a valve body for opening and closing a fluid path in which fluid such as hydrogen, air which contains oxygen or the like flows is used. A back pressure valve as a valve device is provided on a fluid path of cathode off-gas exhausted from a fuel cell and adjusts the pressure of the cathode off-gas exhausted from the fuel cell to a predetermined pressure. As the back pressure valve, for example, a butterfly valve for carrying out opening and closing of a fluid path by means of a disk-shaped valve body is used, and, in order to maintain the set valve opening in stability, it is necessary to retain the rotational direction position of a valve body driving shaft for driving the valve body.
On the other hand, as a locking mechanism for retaining the rotational direction position of a rotational shaft, an off brake is known and disclosed, for example, in Japanese Patent Laid-Open No. 2005-69391. In the off brake, when it is in an off state, the stator side is connected to the rotor side and a rotary shaft mounted on the rotor side is braked for a braking operation. On the other hand, when the off brake is in an on state, the stator side is disconnected from the rotor side to cancel the braking operation for the rotary shaft.
The off brake described presses, upon deenergization wherein the electromagnet is in a non-excitation state, an armature against the rotor side under elastic force of a braking spring to carry out braking operation. However, upon energization wherein the electromagnet is in an excitation state, the armature is attracted and spaced away from the rotor side to cancel the braking operation. With such an electromagnetic brake as just described, a rotational direction position of the rotary shaft can be retained without energization.